1. Field of the Invention
The present invention relates to a loop forming device for a chainstitch sewing, and more particularly to a loop forming device for successively forming the empty rings of yarn by a plurality of needle threads and a looper thread.
2. Description of the Related Art
A well known technique is disclosed in Japanese patent laid-open No. 2000-93671. This technique will be described with reference to FIGS. 8 and 9.
FIG. 8 is a perspective view showing a cloth holding device and a throat plate, and reference numeral 20 designates a support member detachably mounted on the lower end of the presser bar (not shown). Reference numeral 21 designates a cloth holder part, which is rotatably supported at the lower end of the support member 20 by its horizontal shaft 22. Reference numeral 23 indicates a loop holder, which is rotatably supported by a shaft 23A oriented horizontal to the cloth holder part 21.
Reference numeral 24 indicates a throat plate claw plate. Needle location holes 24A, as needle location parts, for three needles N, a gate-shaped slit 24B formed in front of the needle location holes 24A as viewed in the cloth feeding direction, and an edge sewing-up pawl 24C are provided on the throat plate claw plate 24, and its base end is fastened to a throat plate 25.
Three arcuate hollow parts 25A to 25C shown in FIG. 9 are provided at a location under the needle location holes 24A of the throat plate claw plate 24 of the throat plate 25. A thread setter 25F having two engaging claws 25D and 25E which are slanted upward in the cloth feeding direction and provided among the arcuate hollow parts 25A to 25C, is further provided on the throat plate 25. A cloth feed dog 27 through which an auxiliary feed dog 26 appears and disappears and a main feed gear hole 30 through which a main feed dog 29 including a center loop feed dog 28 appears and disappears, are formed in the throat plate 25. Reference numeral 31 is a looper. The width of the loop feed dog 28 in a direction perpendicular to the cloth feeding direction is narrower than the width of an array of the three needles N. Operation of the conventional technique described above will briefly be described.
When the sewing machine is driven, the three needles N vertically moves interlocking with the main shaft (not shown). A looper 31 swings around the needles on a planar shape, while drawing an elliptical locus, and the main feed dog 29 and the auxiliary feed dog 26 perform four movements of cloth feeding.
Motion relationship of those needles N, the looper 31 and the feed dog will be described. The description will start from a state that the needles N are at the bottom dead point, the looper 31 is at the most retracted position, and the feed dog is under the throat plate.
The needles N starts to slightly lift from the bottom dead point. Then the looper 31 starts to advance, the needle threads NT of the three needles N are looped, and those needle threads looped are successively caught by the point of a sword of the looper 31.
The needle thread loop caught by the looper 31 is pulled in the advancing direction of the looper 31. The needle thread loop is retained at the retaining claws 25D and 25E provided among the arcuate hollow parts 25A to 25C as shown in FIG. 9, are set at the arcuate hollow parts 25A to 25C.
The needles N further lift and the looper 31 also proceeds, and the needle threads NT are taken up by a thread take-up lever (not shown).
The needles N further lift and reach the top dead point, and then starts to descend. The looper 31 advances while moving in the direction opposite to the cloth feeding direction, and after it reaches the most advanced position, and then start to retract.
The three needles N having descended, as shown in FIG. 10, are caught by the looper 31, and reaches the lower most point in a state that it is placed in a called triangle of the needle threads NT extending in the cloth feeding direction.
The up and down movement of the needles N and the reciprocal motion of the looper 31 are repeated, and loops 32 as a chain-off thread are formed, as shown in FIG. 9, in the cloth feeding direction by the three needles N and a single looper thread LT. The loops 32 are fed in the cloth feeding direction by a loop feed dog 28 of the main feed dog 29 and the loop holder 23.
The conventional technique, however, involves the following problems.
The loops 32, which is formed in the cloth feeding direction by the three needles N and the single looper thread LT, is swung to the front and rear, and to the right and left by the up/down motion of the needles N and the reciprocating motion of the looper thread L. The needle threads NT that are retained at the retaining claws 25D and 25E as indicated by solid lines in FIG. 9 climb over the retaining claw 25E as indicated by a two-dot chain line, and are retained at the retaining claw 25D. And those threads are unable to return to their original positions, and the subsequent formation of loops is impossible.
The loops 32 fed in the cloth feeding direction are pulled out in the cloth feeding direction by the loop feed dog 28 of the main feed dog 29 and the loop holder 23, and the fore ends of the loops 32 are pulled to the center to be narrow as shown in FIG. 9. A called triangle of the needle threads NT, which are caught by the looper 31 and extend in the cloth feeding direction, becomes small as indicated by a two-dot chain line FIG. 10. The needle does not fall to within the triangle and the stitch skipping occurs, and the loop cannot be formed.
The width of the loop feed dog 28 in a direction orthogonal to the cloth feeding direction is narrow than the width of the needle location part of the three needles N. The fore ends of the loops 32, which are pulled out in the cloth feeding direction to be narrow, are deviated sideways from the loop feed dog 28 as shown in FIG. 11, and loops cannot be fed in the cloth direction, and it is impossible to form the loops.
To achieve the above object, there is provided a chainstitch sewing machine having a plurality of needles having needle threads which are moved up and down through needle holes of a throat plate, a looper having a looper thread, which is reciprocatively moved in a direction orthogonal to a cloth feeding direction under the throat plate, guide holes provided in the throat plate for guiding loops formed by the needle threads and a looper thread by the cooperation of the needles and the looper, to a rear part in a cloth feeding direction, and a main feed dog, disposed at a part closer to a rear than the guide hole, for feeding the loops in the cloth feeding direction,
the improvement being characterized in that
the upper end edge of the needle hole located at a rear part as viewed in the cloth feeding direction, takes a planar shape configured such that a part thereof located closer to the forward movement end of the looper is gradually varied toward the needles with respect to a straight line orthogonal to a cloth feed direction, and a thread guide surface, which is chamfered off to become thin in the forward direction of the looper and toward the lower surface of the throat plate, is provided on the upper edge of the guide hole.
In the chainstitch sewing machine mentioned above, the loops are fed out by a loop holder and a loop feed dog, the loops are expanded, so that the needle threads being in contact with the thread guide surface move along the guide surface and in a direction that the needle threads separate from each other. As a result, the needle threads are caught by the looper, a called triangle of a needle thread NT extending in the cloth feeding direction becomes large. The needle reliably drops into the triangle, and no stitch skipping occurs. Accordingly, the loops are formed reliably.
In the chainstitch sewing machine, the planar shape is arcuate.
With this feature, the effects referred to above is enhanced.
In the chainstitch sewing machine characteristically featured, the thread guide surface may be chamfered to be arcuate.
In the chainstitch sewing machine having the last mentioned feature, a plurality of needle thread loops ranging from the under surface of the looper to the thread guide surface are moved in the cloth feeding direction while contacting with the arcuate chamfered part of the thread guide surface. Therefore, no thread breakage occurs and the loops can reliably be formed.
In the chainstitch sewing machine having any of the features mentioned above, a plurality of thread guide grooves for guiding a plurality of needle thread loops ranging from the under surface of the looper to the thread guide surface are provided in the lower surface of the throat plate located between a needle location part of the throat plate and the thread guide surface of the guide hole.
In the chainstitch sewing machine having the just-mentioned feature, a plurality of needle thread loops ranging from the under surface of the looper to the thread guide surface are reliably guided by the thread guide grooves. This feature ensures reliable formation of the loops.
In the chainstitch sewing machine having the just-mentioned feature, a rear pat of each thread guide groove is arcuate directed downward in the cloth feeding direction.
In the chainstitch sewing machine having this feature, the loops are smoothly retracted, whereby beautiful stitches are formed.
In the chainstitch sewing machine having any of the technical features stated above, the thread guide surface the thread guide grooves closer to on the backward movement end of the looper are located at positions closer to outside than a needle location located closer to the backward movement end of the looper.
In the chainstitch sewing machine having this feature, the needle threads are caught by the looper, a called triangle of a needle thread NT extending in the cloth feeding direction becomes large. The needle reliably drops into the triangle, and no stitch skipping occurs. Accordingly, the loops are formed reliably.
In the chainstitch sewing machine having any of the features stated above, the width of the main feed dog is broader than the width of a plurality of needle location parts of the throat plate.
The chainstitch sewing machine thus constructed is capable of reliably feeding the loops by the main feed dog.